Optical pickup head apparatus

ABSTRACT

An optical pickup head apparatus comprising: a light source; an imaging optics for receiving a linearly polarized light beam emitted from the light source, and converges it on a small spot on an information medium; photodetector units for receiving the light beam being reflected and diffracted at the information medium and output electric signals corresponding to received light quantities; a liquid crystal hologram consisting of a pair of transparent substrates, a liquid crystal sandwiched therebetween through a pair of transparent electrodes, either of the transparent electrodes being formed with a hologram pattern; and a Faraday rotator for rotating a polarized direction of light beam by 45 degrees in a light path leading from the light source to the information medium and, further, rotates the polarized direction of the light beam by 45 degrees in its opposite light path. The liquid crystal hologram is applied a constant voltage so that the light beam reflected at the information medium is diffracted in the liquid crystal hologram to generate ±1-order diffraction light beams, and the photodetector units sense both of light quantities of said ±1-order diffraction light beams emitted from the liquid crystal hologram. Then, a subtraction between signals representing the light quantities is calculated so as to obtain an information signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical pickup head for opticallyrecording, reproducing, or erasing information on or from an optical ormagneto-optical medium such as an optical disk or an optical card.

2. Description of the Prior Art

An optical memory technology using an optical disk having pit-patternhas been expanding its utilization field as a high-density andlarge-capacity recording medium, so as to be used as a digital audiodisk, a video disk, a floppy disk, and further a data file.

For such an optical memory technology, it is important to accuratelycarry out the recording on or reproducing from the optical disk throughoptical beam squeezed in a thin beam with high reliability. Thismechanism largely depends on its optical system in reliability.

An optical head generally serves as an essential part of the opticalsystem. Basic functions of the optical head are roughly categorized intoa convergence for forming a diffraction-limited small spot, a focusingservo and tracking control in the optical system, and pit signal(information signal) detection.

These functions are embodied by various combinations of optical systemsand photo-electrical conversion detecting systems on the basis of theirpurposes and uses. Especially, an optical pickup head apparatus usinghologram has been recently introduced in order to reduce the size ofoptical pickup head apparatus itself and manufacture it thin.

As an example of prior art, FIGS. 24 and 25 show a constitution of anoptical pickup head disclosed by J. C. LEHUREAU, J. Y. BEGUIN and J.COLINEAU; "Polarizing Grating Beamsplitter Using a Liquid Crystal Cell",Proc. Int. Symp. on Optical Memory, 1989 Japanese Journal of AppliedPhysics, Vol. 28 (1989) Supplement 28-3, pp. 201-203.

In FIG. 24, a reference numeral 2 denotes a radiation light source; forexample, a semiconductor laser. A linearly polarized light beam 3 (alaser beam) emitted from this light source 2 passes through a liquidcrystal hologram 172 and, in turn, is converted into a circularpolarized beam by means of a quarter wavelength plate 15. Then, thecircular polarized beam enters into an objective lens 4 and convergesonto an information medium 5.

After reflection at the information medium 5, the rotational directionof the circular polarized beam is reversed. Then the light beam 3travels the same light path in an opposite direction, and enters againinto the quarter wavelength plate 15. In the quarter wavelength plate15, the light beam 3 becomes a linearly polarized beam with a polarizeddirection rotated 90 degrees from its initial direction.

Subsequently, the light beam 3 enters into the liquid crystal hologram172, and from which a +1-order diffraction light beam 66 inters into aphotodetector unit 7. By calculating outputs of the photodetector unit7, servo signals (i.e. focus error signals and tracking error signals)and information signals can be obtained.

As shown in FIG. 25, the liquid crystal hologram 172 consists of a pairof transparent substrates (glasses) 9 and 9, a polyimide 23 for forminga brazed hologram, a liquid crystal 17, and a pair of transparentelectrodes 16 and 16.

An agent for orientating liquid crystal molecules and a sealing materialare often used for the liquid crystal hologram 172 but are not relatedto the present invention. Therefore, they are not shown in the drawing.The liquid crystal 17 includes elliptic liquid crystal molecules 17ahaving a refractive index n_(s) in its minor-axis direction and arefractive index n_(l) in its major-axis direction. In this case, therefractive index n_(s) is selected to be substantially the same as arefractive index n_(p) of the polyimide 23 and the refractive indexn_(l) is largely different from the refractive index n_(p).

When a light beam 3 being linearly polarized in the minor axis direction(i.e. direction 1) of the liquid crystal 17 oriented in this liquidcrystal hologram 172 is entered into the liquid crystal hologram 172, nodiffraction is generated since the polyimide 23 and the liquid crystal17 have substantially the same refractive index (n_(s) ≈n_(p)).

On the contrary, when a light beam 3 being linearly polarized in adifferent direction (i.e. direction 2) normal to the above-describedpolarized direction is entered into the liquid crystal hologram 172, adiffraction is generated due to a refractive index difference betweenthe liquid crystal 17 and the polyimide 23 (n_(l) ≠n_(p)). Furthermore,since the polyimide 23 is brazed as shown in FIG. 25, +1-orderdiffraction light beam becomes strong.

By utilizing above-described property of such a liquid crystal hologram172, if the light beam 3 is emitted in FIG. 24 so that a polarizeddirection of the light beam 3 becomes parallel with the direction 1, nodiffraction occurs in a light beam path (forward light beam path)leading from the radiation light source 2 to the information medium 5but the diffraction efficiency of the +1-order diffraction light beambecomes high in its opposite light beam path (backward light beam path)leading from the information medium 5 to the radiation light source 2.

Accordingly it is concluded that the efficiency of use of light beam inthe forward and backward light beam paths is high. Hereupon, theefficiency of use of light beam in the forward and backward light beampaths is defined in general by multiplying the light quantity (0-orderdiffraction light quantity) passing through the hologram 172 in theforward light beam path and the +1-order diffraction light quantityemitted from the hologram in the backward light beam path.

According to this prior art, a focusing error (FE) signal is sensed byFoucault method or by astigmatic method. However, in the case where theFoucault method is adopted, a mechanical knife edge 22 shown in FIG. 24must be additionally installed. On the other hand, in the case where theastigmatic method is adopted, a cylindrical lens must be additionallyinstalled. In any case, there was a problem that the number of partsincreased and therefore it resulted in cost up.

One method for solving above-described problem is to give a curvature ona hologram curve of the liquid crystal hologram 172. However, as ismentioned in the above-introduced reference paper, it is known thatmanufacturing a brazed hologram to have a curvature on its hologrampattern has been quite difficult.

Furthermore, this prior art mentions that the liquid crystal hologramcan be generally used as a substitution for one polarized beamsplitterin an optical pickup head apparatus for a magneto-optical disk requiringmore than two polarized beamsplitters. However, this is only effectivein reducing a size of one component, and does not result in a sufficientoverall reduction in size of optical pickup head apparatus.

SUMMARY OF THE INVENTION

Accordingly, the present invention has a purpose to provide an opticalpickup head apparatus which can be constituted by a minimum number ofparts, can be manufactured compact in size and light in weight at lowcost, and can detect a change of polarized angle caused by not only aread-only optical disk but also by a magneto-optical disk.

In order to accomplish above purposes, a first aspect of the presentinvention provides an optical pickup head apparatus comprising: aradiation light source; an imaging optics which receives a linearlypolarized light beam emitted from said radiation light source, andconverges the linearly polarized light beam to a small spot on aninformation medium; photodetector units including a plurality ofphotodetectors which receive the light beam being reflected anddiffracted at the information medium and output electric signals inaccordance with received light quantities; a liquid crystal hologramwhich is constituted by a pair of transparent substrates, a liquidcrystal sandwiched between these transparent substrates, and a pair oftransparent electrodes disposed between the liquid crystal and thetransparent substrates, either of said transparent electrodes beingformed with a hologram pattern;

a quarter wavelength plate which converts said linearly polarized lightbeam into a circular polarized light beam in a forward light pathleading from said radiation light source to said information medium and,to the contrary, restores thus converted circular polarized light beaminto a linearly polarized light beam in a backward light path leadingfrom said information medium to the radiation light source;

said liquid crystal hologram being applied a constant voltage so thatsaid light beam reflected at the information medium is diffracted in theliquid crystal hologram to generate ±1-order diffraction light beams;and

said photodetector units sensing both of light quantities of said±1-order diffraction light beams emitted from the liquid crystalhologram and calculating a summation between signals representing saidlight quantities so as to obtain an information signal.

And, a second aspect of the present invention provides an optical pickuphead apparatus comprising: a radiation light source; an imaging opticswhich receives a linearly polarized light beam emitted from saidradiation light source, and converges the linearly polarized light beamto a small spot on an information medium; photodetector units includinga plurality of photodetectors which receive the light beam beingreflected and diffracted at the information medium and output electricsignals in accordance with received light quantities; a liquid crystalhologram which is constituted by a pair of transparent substrates, aliquid crystal sandwiched between these transparent substrates, and apair of transparent electrodes disposed between the liquid crystal andthe transparent substrates, either of said transparent electrodes beingformed with a hologram pattern;

one transparent substrate located closer to said information medium ofsaid two transparent substrates constituting the liquid crystal hologrambeing made by a quarter wavelength plate which converts said linearlypolarized light beam into a circular polarized light beam in a forwardlight path leading from said radiation light source to said informationmedium and, to the contrary, restores thus converted circular polarizedlight beam into a linearly polarized light beam in a backward light pathleading from said information medium to the radiation light source;

said liquid crystal hologram being applied a constant voltage so thatsaid light beam reflected at the information medium is diffracted in theliquid crystal hologram to generate ±1-order-diffraction light beams;and

said photodetector units sensing both of light quantities of said±1-order diffraction light beams emitted from the liquid crystalhologram and calculating a summation between signals representing saidlight quantities so as to obtain an information signal.

Furthermore, a third aspect of the present invention provides an opticalpickup head apparatus comprising: a radiation light source; an imagingoptics which receives a linearly polarized light beam emitted from saidradiation light source, and converges the linearly polarized light beamto a small spot on an information medium; photodetector units includinga plurality of photodetectors which receive the light beam beingreflected and diffracted at the information medium and output electricsignals in accordance with received light quantities; a liquid crystalhologram which is constituted by a pair of transparent substrates, aliquid crystal sandwiched between these transparent substrates, and apair of transparent electrodes disposed between the liquid crystal andthe transparent substrates, either of said transparent electrodes beingformed with a hologram pattern;

a Faraday rotator which rotates a polarized direction of said light beamby 45 degrees in a forward light path leading from said radiation lightsource to said information medium and, further, rotates the polarizeddirection of said light beam by 45 degrees in a backward light pathleading from said information medium to the radiation light source;

said liquid crystal hologram being applied a constant voltage so thatsaid light beam reflected by the information medium is-diffracted in theliquid crystal hologram to generate ±1-order diffraction light beams;and

said photodetector units sensing both of light quantities of said±1-order diffraction light beams emitted from the liquid crystalhologram and calculating a subtraction between signals representing saidlight quantities so as to obtain an information signal.

Moreover, a fourth aspect of the present invention provides an opticalpickup head apparatus comprising: a radiation light source; an imagingoptics which receives a linearly polarized light beam emitted from saidradiation light source, and converges the linearly polarized light beamto a small spot on an information medium; photodetector units includinga plurality of photodetectors which receive the light beam beingreflected and diffracted at the information medium and output electricsignals in accordance with received light quantities; a liquid crystalhologram which is constituted by a pair of transparent substrates, aliquid crystal sandwiched between these transparent substrates, and apair of transparent electrodes disposed between the liquid crystal andthe transparent substrates, either of said transparent electrodes beingformed with a hologram pattern;

one of said two transparent substrates constituting the liquid crystalhologram being made by a dielectric substrate serving as one componentof a Faraday rotator which rotates a polarized direction of said lightbeam by 45 degrees in a forward light path leading from said radiationlight source to said information medium and, further, rotates thepolarized direction of said light beam by 45 degrees in a backward lightpath leading from said information medium to the radiation light source;

said liquid crystal hologram being applied a constant voltage so thatsaid light beam reflected at the information medium is diffracted in theliquid crystal hologram to generate ±1-order diffraction light beams;and

said photodetectors sensing both of light quantities of said ±1-orderdiffraction light beams emitted from the liquid crystal hologram andcalculating a subtraction between signals representing said lightquantities so as to obtain an information signal.

Still further, a fifth embodiment of the present invention provides anoptical pickup head apparatus comprising: a radiation light source; animaging optics which receives a linearly polarized light beam emittedfrom said radiation light source, and converges the linearly polarizedlight beam to a small spot on an information medium; photodetector unitsincluding a plurality of photodetectors which receive the light beambeing reflected and diffracted at the information medium and outputelectric signals in accordance with received light quantities; a liquidcrystal hologram which is constituted by a pair of transparentsubstrates, a liquid crystal sandwiched between these transparentsubstrates, and a pair of transparent electrodes disposed between theliquid crystal and the transparent substrates, either of saidtransparent electrodes being formed with a hologram pattern;

a Faraday rotator which rotates a polarized direction of said light beamby 45 degrees in a forward light path leading from said radiation lightsource to said information medium and, further, rotates the polarizeddirection of said light beam by 45 degrees in a backward light pathleading from said information medium to the radiation light source;

said liquid crystal hologram being applied a high voltage in the casewhere information signal is read out higher than the case whereinformation signal is written in, so that said light beam reflected atthe information medium is diffracted in the liquid crystal hologram togenerate ±1-order diffraction light beams; and

said photodetectors sensing both of light quantities of said ±1-orderdiffraction light beams emitted from the liquid crystal hologram andcalculating a subtraction between signals representing said lightquantities so as to obtain the information signal.

Yet further, a sixth aspect of the present invention provides an opticalpickup head apparatus comprising: a radiation light source; an imagingoptics which receives a linearly polarized light beam emitted from saidradiation light source, and converges the linearly polarized light beamto a small spot on an information medium; photodetector units includinga plurality of photodetectors which receive the light beam beingreflected and diffracted at the information medium and output electricsignals in accordance with received light quantities; a liquid crystalhologram which is constituted by a pair of transparent substrates, aliquid crystal sandwiched between these transparent substrates, and apair of transparent electrodes disposed between the liquid crystal andthe transparent substrates, either of said transparent electrodes beingformed with a hologram pattern;

one of said two transparent substrates constituting the liquid crystalhologram being made by a dielectric substrate serving as one componentof a Faraday rotator which rotates a polarized direction of said lightbeam by 45 degrees in a forward light path leading from said radiationlight source to said information medium and, further, rotates thepolarized direction of said light beam by 45 degrees in a backward lightpath leading from said information medium to the radiation light source;

said liquid crystal hologram being applied a high voltage in the casewhere information signal is read out higher than the case whereinformation signal is written in, so that said light beam reflected atthe information medium is diffracted in the liquid crystal hologram togenerate ±1-order diffraction light beams; and

said photodetectors sensing both of light quantities of said ±1-orderdiffraction light beams emitted from the liquid crystal hologram andcalculating a subtraction between signals representing said lightquantities so as to obtain the information signal.

With these arrangements, the optical pickup head can be constituted bythe liquid crystal hologram and the quarter wavelength plate or theFaraday rotator, which results in improvement in utilization efficiencyand results in S/N ratio of a servo signal or an information signal.

The liquid crystal hologram adopted in the present invention can beobtained by merely pattering transparent electrodes to form a hologrampattern by use of photo-mask manufactured as an application of anintegrated circuit technology. Namely, no manufacturing of brazedcross-sectional configuration is required. This is advantageous in makethe hologram pattern have curvature, or in exchanging the hologrampattern partly with a different hologram pattern.

The present invention utilizes the fact that, when a certain valuevoltage is applied to the liquid crystal hologram, the +1-orderdiffraction light beam and the -1-order diffraction light beam changecomplementarily in accordance with a polarization angle of incidentlight beam. That is, a polarization angle change; i.e. an informationsignal of magneto-optical disk, can be detected by the subtractionbetween an electric signal generated from the photo-detector uponreceipt of the +1-order diffraction light beam and an electric signalgenerated from the photo-detector upon receipt of the -1-orderdiffraction light beam.

Thus obtained information signal is advantageous in its stability. Thisis, this information signal is derived from differential detectionbetween the +1-order diffraction light beam and the -1-order diffractionlight beam. Therefore, bad affection of light quantity variation causedby the change of the radiation light source itself or by flaw of theinformation medium can be canceled by the differential nature.

Still further, an optical pickup head apparatus capable of detecting thepolarization angle change by the magnetic disk can be constituted at lowcost by use of the minimum number of components, so as to realize alight weight device.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription which is to be read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an optical pickuphead apparatus in accordance with a first embodiment of the presentinvention;

FIG. 2 is a perspective view showing a liquid crystal hologram servingas one component of the optical pickup head apparatus of the firstembodiment of the present invention;

FIGS. 3(a)-3(c) are illustrative views showing orientations of liquidcrystal molecules in the case where a low voltage is applied to theliquid crystal hologram of the first embodiment of the presentinvention;

FIG. 4 is a graph showing changes of diffraction efficiencies in thecase where various voltages are applied to the liquid crystal hologramof the first embodiment of the present invention;

FIGS. 5(a)-5(b) are illustrative views showing orientations of liquidcrystal molecules in the case where a high voltage is applied to theliquid crystal hologram of a second embodiment of the present invention;

FIG. 6 is a graph showing changes of diffraction efficiencies in thecase where a high voltage is applied to the liquid crystal hologram ofthe second embodiment of the present invention;

FIG. 7 is a schematic cross-sectional view showing an optical pickuphead apparatus in accordance with the second embodiment of the presentinvention;

FIG. 8 is a perspective view showing a liquid crystal hologram servingas one component of the optical pickup head apparatus of a thirdembodiment of the present invention;

FIG. 9 is a schematic cross-sectional view showing an optical pickuphead apparatus in accordance with the third and fourth embodiments ofthe present invention;

FIG. 10 is a perspective view showing a liquid crystal hologram servingas one component of the optical pickup head apparatus of the fourthembodiment of the present invention;

FIG. 11 is a plane view showing a hologram pattern of the liquid crystalhologram in accordance with embodiments of the present invention;

FIG. 12 is a schematic cross-sectional view of the optical pickup headfor illustrating diffraction light beams (spherical waves);

FIGS. 13(a)-13(c) are plane views showing the diffraction light beams onthe photodetectors;

FIG. 14 is a schematic perspective view showing essential parts(hologram patterns and photodetectors) of the optical pickup headapparatus in accordance with embodiments of the present invention;

FIG. 15 is a plane view showing the diffraction light beams on thephotodetectors;

FIG. 16 is a schematic cross-sectional view showing one modifiedembodiment of the optical pickup head apparatus of FIG. 1;

FIG. 17 is a schematic cross-sectional view showing one modifiedembodiment of the optical pickup head apparatus of FIG. 7;

FIG. 18 is a schematic cross-sectional view showing one modifiedembodiment of the optical pickup head apparatus of FIG. 9;

FIG. 19 is a schematic cross-sectional view showing another modifiedembodiment of the optical pickup head apparatus of FIG. 1;

FIG. 20 is a schematic cross-sectional view showing another modifiedembodiment of the optical pickup head apparatus of FIG. 7;

FIG. 21 is a schematic cross-sectional view showing another modifiedembodiment of the optical pickup head apparatus of FIG. 9;

FIG. 22 is a schematic perspective view showing a module in whichphotodetector units and a radiation light source are integrally mounted;

FIG. 23 is a schematic cross-sectional view showing an optical pickuphead apparatus in accordance with a fifth embodiment of the presentinvention;

FIG. 24 is a schematic cross-sectional view showing a conventionaloptical pickup head apparatus;

FIGS. 25(a)-25(b) are a schematic cross-sectional views showing a liquidcrystal hologram serving as one component of the conventional opticalpickup head apparatus.

DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, referring now to the accompanying drawings, embodiments ofthe present invention are explained in detail.

FIRST EMBODIMENT

FIG. 1 is a view showing a constitution of a first embodiment of thepresent invention. A reference numeral 2 denotes a radiation lightsource such as a semiconductor laser or the like. On a forward lightpath leading from the light source 2 to an information medium 5, alinearly polarized light beam 3 (a laser light beam) emitted from thislight source 2 passes through a liquid crystal hologram 170 and, inturn, is converted into a circular polarized light beam 3 by means of aquarter wavelength plate 15 to enter an objective lens 4. Then, afterhaving passed through the objective lens 4, the light beam 3 isconverged on the information medium 5. That is, the objective lens 4constitutes an imaging optics of the present invention.

When the light beam 3 is reflected at the information medium 5, itsrotational direction of the circular polarized light beam 3 is reversed.On a backward light path leading from the information medium 5 towardthe light source 2, the light beam 3 returns along the same light pathas the forward light path. Upon passing through the quarter wavelengthplate 15, the light beam 3 is restored into a linearly polarized lightbeam 3 being rotated by total 90 degrees from its initial condition.Then, the light beam 3 enters into the liquid crystal hologram 170.

The liquid crystal hologram 170 serves to make the light beam 3 emit+1-order diffraction light beam 64 and -1-order diffraction light beam65. These +1-order diffraction light beam 64 and -1-order diffractionlight beam 65 are inputted into photodetector units 100 and 101,respectively. By calculating outputs from the photodetector units 100and 101, servo-signals and information signals can be obtained.

Especially, in the case where information are read out on the basis of achange of incident light quantity reflected from the information medium5 to the objective lens 4, it is possible to detect information signalsby sensing these +1-order diffraction light beam 64 and -1-orderdiffraction light beam 65 in the photodetector units 100 and 101,respectively, and thereafter obtaining a summation of outputs from thesephotodetector units 100 and 101, or obtaining an alternating-currentcomponent of thus calculated summation of the outputs.

As shown in FIG. 2, the liquid crystal hologram 170 consists of a pairof transparent substrates (for example, glasses) 9 and 9, and a liquidcrystal 17 interposed between these two transparent substrates through apair of transparent electrodes 16 and 16. Namely, the liquid crystal 17is sandwiched by the transparent substrates 9, 9 through transparentelectrodes 16, 16 in an up-and-down direction. Though an agent fororientating liquid crystal molecules and a sealing material are oftenused for the liquid crystal hologram, they are not directly related tothe present invention and therefore omitted in the drawing.

In this embodiment, either of the transparent electrodes 16, 16 isformed with a hologram pattern. FIGS. 3(a)-3(c) illustrate variations ofliquid crystal molecules in the case where the transparent electrodes16, 16 are applied a voltage, when seen from both of the top (i.e.Z-direction) and the side (i.e. Y-direction) of the liquid crystalhologram 170 shown in FIG. 2. In FIGS. 3(a)-3(c), reference numerals 21,21a, and 21b show typical orientation directions of liquid crystalmolecules.

When no voltage is applied, liquid crystal molecules are oriented in thesame direction by use of a rubbing or orientation agent, as shown inFIG. 2(a). On the other hand when a significant voltage is applied, onlythe molecules with their both sides sandwiched by the transparentelectrodes are affected by this voltage application such that theirorientating directions are changed as shown in FIG. 2(b). Namely, theliquid crystal molecules applied the significant voltage are disposed toarray their major axes along the up-and-down (Z-axis) direction. In thedrawing, a dot-point (.) shows that its corresponding liquid crystalmolecule stands vertically on the paper surface.

Under the condition shown in FIG. 2(b), if a linearly polarized lightbeam enters in the Y-direction (i.e. 0-degree polarized direction), adiffraction is generated due to refractive index difference between themajor axis and the minor axis of the liquid crystal molecule since theliquid crystal molecules are arrayed as shown by reference numerals 21aand 21b in FIG. 3(b).

On the contrary, if a linearly polarized light beam enters in theX-direction (i.e. 90-degree polarized direction), a diffraction is notcaused because the light beam is influenced only by theminor-axis-direction refractive indexes of the molecules 21a and 21b inthe condition of FIG. 3(b).

Therefore, as is explained above, it will be understood that the liquidcrystal hologram of the present invention has a polarizing anisotropy aswell as the conventional hologram.

FIG. 4 shows an experimental result of diffraction efficiencies measuredby supplying a linearly polarized laser beam into an actuallymanufactured liquid crystal hologram while applying an AC voltage of 3KHz to this liquid crystal hologram. A reference symbol Θ denotes apolarized angle. An abscissa represents an applied voltage value, and anordinate represents a diffraction efficient of 1-order diffraction lightbeam.

According to this experimental result, it was found that the highestquenching ratio is obtained when a voltage of approximately 3 V isapplied to the liquid crystal hologram.

In FIG. 1, if the polarized direction of the light beam 3 is set towardthe X-direction (90-degree polarized direction) of FIGS. 3(a)-3(c), nodiffraction occurs in the forward light path leading from the radiationlight source 2 to the information medium 5 by virtue of theabove-described property of liquid crystal hologram 170.

However, on the backward light path, since the polarized direction isrotated 90 degrees (0-degree polarized direction) from the polarizeddirection of the incident light, the hologram 170 causes a diffraction.In other words, the diffraction efficiencies of ±1-order diffractionlight beams become high.

Accordingly, extremely high is the efficiency of use of light beam inthe forward and backward light beam paths which is defined by amultiplication of the light quantity (0-order diffraction lightquantity) passing through the hologram 170 in the forward light beampath and the ±1-order diffraction light quantities emitted from thehologram 170 in the backward light beam path. Therefore, by utilizing asummation of signals obtained from the ±1-order diffraction light beamsas an information signal, it becomes possible to constitute an opticalpickup head having a high S/N ratio as an optical pickup head apparatusfor a read-only or a phase modulation type optical disk.

The first embodiment is mainly explained based on a liquid crystalhologram. However, it should be noted that the present invention is notlimited to the liquid crystal hologram. The reason why the liquidcrystal hologram is selected as an element of the first embodiment isthat it has polarizing anisotropy as explained above. Thus, it is easilyunderstood that the liquid crystal hologram disclosed in the embodimentcan be replaced by any other hologram having polarizing anisotropicnature, which is so-called "polarizing hologram beamsplitter". Forexample, lithium niobate, which has an anisotropic refractive index,would be used as a hologram element of the present invention when thecrystal surface is proton exchanged so as to obtain a lattice formation.

SECOND EMBODIMENT

Hereinafter, described is an example for optically reading out, writing,or erasing information signals from or on a magneto-optical disk by useof the liquid crystal hologram 170 as a second embodiment of the presentinvention.

In the second embodiment, the liquid crystal hologram 170 is appliedwith a voltage higher than the first embodiment. In this case, anelectric field 19 applied to the liquid crystal 17 in the liquid crystalhologram 170 is formed in a chevron shape as shown in FIG. 5(a).

For this reason, the liquid crystal molecule 21 becomes oblique whenseen from the beam direction, as shown in FIG. 5(b). Since the liquidcrystal molecules are disposed obliquely, the diffraction efficiencieswith respect to conjugate waves (i.e. +1-order diffraction light beamand -1-order diffraction light beam) become asymmetrical. Namely, thesame effect as the brazed effect can be obtained in this embodiment. Anexperimental result relating to this embodiment is shown in FIG. 6.

FIG. 6 is a graph showing an experimental result of ±1-order diffractionefficiencies measured by supplying a linearly polarized laser beam intothe liquid crystal hologram shown in FIG. 2 while applying an AC voltageof 6 V at 3 KHz to this liquid crystal hologram. An abscissa representsa polarization angle, and an ordinate represents a diffractionefficient.

From this graph, it would be understood that the diffractionefficiencies of the +1-order diffraction light beam and the -1-orderdiffraction light beam change complementarily in accordance with apolarization angle of an incident light beam.

A fundamental feature of the present invention is to realize an opticalpickup head apparatus capable of detecting a polarization angle changecaused by a magnetic disk; i.e. information signal, at low cost by useof the minimum number of components, so as to realize a light weightdevice.

In an optical pickup head apparatus shown in FIG. 7, if the polarizeddirection of the light beam 3 is set toward the X-direction (Θ=±90degrees) of FIG. 5, little diffraction occurs in the forward light pathleading from the radiation light source 2 to the information medium 5 byvirtue of the above-described property of liquid crystal hologram 170.Then, after having passed through the liquid crystal hologram 170, thelight beam 3 is rotated 45 degrees in its polarized direction by meansof a Faraday rotator 151.

Subsequently, on its opposite light path (i.e. on the backward lightpath), the light beam 3 is further rotated 45 degrees in its polarizeddirection. Therefore, when the light beam 3 returns to enter into theliquid crystal hologram 170, the polarized direction of the light beam 3becomes Y-direction (i.e. Θ=0 degree) in FIG. 5. Accordingly, thediffraction efficiencies of the ±1-order diffraction light become high.Thus, an optical pickup head having a high efficiency in utilizing lightas well as a high S/N ratio can be realized.

By the way, as is well known, this Faraday rotator 181 can be easilyfabricated by a conventional method such as a combination of a magnetand an iron garnet of bismuth-substituted rare earth element.

Furthermore, in the case where a magneto-optical disk is used as theinformation medium 8, when the light beam hits a pit formed on aninformation surface of the information medium 5, the polarized directionis rotated due to magneto-optical effect. Then, on the backward lightpath, when the light beam passes the Faraday,rotator 151, the polarizeddirection is offset from 0 degree.

With such a polarization angle change of the light beam, a returninglight beam from the information medium 5 has a polarization angle changebeing changed in accordance with an information signal. Therefore, thispolarization angle change is shown by a difference signal between anelectric signal obtained from the photodetector unit 100 upon receipt of+1-order diffraction light beam and an electric signal obtained from thephotodetector unit 101 upon receipt of -1-order diffraction light beamor by an AC component of this difference signal, as shown in FIG. 6 or7.

Thus obtained information signal is advantageous in its stability. Thisis, this information signal is derived from differential detectionbetween the +1-order diffraction light beam and the -1-order diffractionlight beam. Therefore, bad affection of light quantity variation causedby the change of the radiation light source itself or by flaw of theinformation medium can be canceled by this differential nature.

In the second embodiment, if it required to increase an optical outputof the information medium 5 in the case where information is written inor erased, it brings a best result to lower a voltage applied to theliquid crystal hologram 170 when information is written in and erasedand, to the contrary, to increase a voltage applied to the liquidcrystal hologram 170 when information is read out.

For instance, in case of the liquid crystal hologram used in thepreviously described experiment, by applying a voltage of approximately3 V in writing or erasing information, the diffraction efficiency can belowered in the forward light path leading from the radiation lightsource 2 to the information medium 5 (with a 90-degree polarizeddirection) as can be understood from FIG. 4. Namely, by increasing apermeability on the forward light path, the optical output from lens canbe increased.

On the other hand, when information is read out, the voltage applied tothe liquid crystal hologram 170 is increased up to approximately 6 V.Thus, information is read out from the magneto-optical disk aspreviously described.

In accordance with this embodiment, the permeability of the forwardlight path can be increased by applying a lower voltage to the liquidcrystal hologram in order to ensure a writing in or erasing operation ofinformation to a recording (information) medium. Accordingly, a lightquantity of light spot for writing or erasing on the information mediumcan be increased.

To the contrary, in the case where information is read out, a highervoltage is applied to the liquid crystal hologram so that the liquidcrystal hologram has the same effect as the brazed effect. Thus,information signals can be read out stably even when an optical pickuphead apparatus constituted by minimum number of parts is used inassociation with a magneto-optical disk.

THIRD EMBODIMENT

FIG. 8 shows a third embodiment of the present invention. In thisembodiment, a quarter wavelength plate 15 is used in place of one oftransparent substrates 9 shown in FIG. 2 which is provided closer to theobjective lens 4. Namely, the quarter wavelength plate 15 of FIG. 1needs not to be specially provided in this embodiment since the liquidcrystal hologram 171 can serve as the quarter wavelength plate as well.

With this arrangement, as shown in FIG. 9, overall part number can befurther reduced to constitute a small-sized, light-weight, and low-costoptical pickup head apparatus, while ensuring the same performance asthe first embodiment.

FOURTH EMBODIMENT

FIG. 10 shows a fourth embodiment of the present invention. In thisembodiment, a dielectric substrate constituting a Faraday rotator 151 isused in place of one of transparent substrates 9 shown in FIG. 2 whichis provided closer to the objective lens 4. Namely, the Faraday rotator151 of FIG. 1 needs not to be specially provided in this embodimentsince the liquid crystal hologram 171 can serve as the Faraday rotatoras well. That is, in the same manner as the third embodiment, with thisarrangement, overall part number can be further decreased to constitutea small-sized, light-weight, and low-cost optical pickup head apparatus,while ensuring the same performance as the second embodiment.

By the way, in FIG. 10, magnets 151a and 151b serve as measures forapplying a magnetic field required for causing a Faraday rotation.However, it is needless to mention that these magnets can-be substitutedby any other measures which can generate the required magnetic field.

FOCUSING ERROR (FE) SIGNAL DETECTION

The following description explains an embodiment for realizingdetections of a focusing error (FE) signal and a tracking error (TE).FIG. 11 shows a typical hologram pattern of the transparent electrodeconstituting the liquid crystal hologram in accordance with the presentinvention. In FIG. 11, reference numerals 153 and 154 denote dividedhologram regions for generating diffraction lights in order to detectthe TE signals, and a reference signal 155 denotes a divided hologramregion for generating a diffraction light in order to detect the FEsignal.

The hologram patten constituting the region 155 can be produced througha computer generated hologram (CGH) method which carries out acalculation for obtaining interference fringes on the liquid crystalhologram 171 of FIG. 12 formed by interference between the light beam 3and the +1-order diffraction light beam 64 or interference between thelight beam 3 and the -1-order diffraction light beam 65.

Here, the +1-order diffraction light beam 64 is a spherical wave havingits focal point behind the photodetector unit 100; i.e. at a point abelow the photodetector unit 100 in FIG. 12. On the other hand, the-1-order diffraction light beam 65 is a spherical wave having its focalpoint in front of the photodetector- unit 101; i.e. at a point b abovethe photodetector unit 101.

FIG. 13 shows the +1-order diffraction light beam 64 and the -1-orderdiffraction light beam 65 detected on the photodetector units 100 and101, respectively. FIG. 13(b) shows a just focus condition, and FIGS.13(a) and 13(c) show defocus conditions. Accordingly, the focus errorsignal FE can be calculated as follows:

    FE=(S1-S2+S3)-(S4-S5+S6)                                   (1)

or

    FE=S5-S2                                                   (2)

In general, when the light beam 3 emitted from the radiation lightsource 2 causes a variation in its wavelength, the diffraction lightbeams 64 and 65 shift toward X-direction in FIG. 13. However, such shiftmovements of diffraction light beams 64 and 65 do not affect the FEsignal obtained in accordance with above-described calculation.

Although, for the purpose of simplifying the explanation, twodiffraction light beams 64, 65 in FIGS. 12 and 13 are depicted asspherical wave, it would be understood from FIGS. 12, 13 and theequations (1), (2) that essentially required are the diffraction lightbeam 65 converging in front of the photodetector unit 101 and itsconjugate diffraction light beam 64 converging behind the photodetectorunit 100. Therefore, it is needless to say that light beams having focallines instead of focal points may be used in this invention.

TRACKING ERROR (TE) SIGNAL DETECTION

Next, an embodiment for detecting the TE signal is explained. In FIG.14, reference numerals 153 and 154 denote diffraction light beamgenerating regions for detecting the TE signals of the hologram patternas well as FIG. 11. Further, a reference numeral 72 denotes aphotodetector for detecting the TE signal.

A light beam reflected at the information medium 5 has a diffractionpattern being formed when the light beam is diffracted by track grooveson the information medium 5. This diffraction pattern is referred to asa far-field pattern 18.

With this far-field pattern 18, a light quantity distribution on thehologram is varied in response to a change of mutual position betweenthe converging spot and the track groove on the information medium 5.

For example, if it is supposed that the Y-direction of FIG. 14 isparallel with the track grooves, the change of light quantity occurs insuch a manner that the far-field pattern of +X-direction (18a) becomesbright and the far-field pattern of -X-direction (18b) becomes dark or,on the contrary, the far-field pattern of +X-direction (18a) becomesdark and the far-field pattern of -X-direction (18b) becomes bright.

Accordingly, the TE signal can be obtained in the tracking error signaldetecting photodetectors 72 by sensing the diffraction light beams 163fed from the diffraction regions 153 and 154.

FIG. 15 shows diffraction light beams on the photodetector units 100 and101. In FIG. 15, detecting portions S7 to S10 correspond to the trackingerror signal detecting photodetector units 72 in FIG. 14. In this case,the tracking error signal TE can be calculated as follows:

    TE=(S7+S9)-(S8+S10)                                        (3)

As is described previously there was a problem in the conventionalsystem such that the polyimide layer needs to be formed to have a brazedcross-sectional configuration as shown in FIG. 25(a) and therefore itwas difficult even to provide the hologram curves, however the presentinvention does not require to fabricate the brazed cross-sectionalconfiguration. That is, the liquid crystal hologram adopted in thepresent invention can be obtained by merely patterning transparentelectrodes to form a hologram pattern by use of photo-mask manufacturedas an application of an integrated circuit technology. This isadvantageous in making the hologram pattern have curvature, or inexchanging the hologram pattern partly with a different hologrampattern.

By the way, it is desirable to provide transparent electrodes on theboundary lines 156 sectioning hologram segment regions 153, 154, and 155so that all the transparent electrodes of the hologram segment regionscan be applied with voltages.

VARIOUS MODIFICATIONS OF FIRST TO FOURTH EMBODIMENTS

Though the embodiments shown in FIGS. 1, 7, and 9 disclose finiteoptical systems which do not adopt collimator lenses so as to reduce thesize and part number of the optical pickup head apparatus, the presentinvention can be realized even if the collimator lenses 41 are disposedbetween the liquid crystal hologram 170 or 171 and the radiation lightsource 2 to change the imaging optics to an infinite system, as shown inFIGS. 16, 17, and 18. With this arrangement, no aberration is generatedin the diffraction light beam even if wavelength variation occurs.Furthermore, this arrangement is advantageous in that the diffractionefficiencies become uniform since substantially parallel light beamsenter the liquid crystal hologram.

Next, the objective lens 4 and the liquid crystal hologram 170 or 171 inthe embodiments of FIGS. 1, 7, and 9 can be assembled in a holder 42 soas to be shifted together, as shown in FIGS. 19, 20, and 21. With thisarrangement, no light beam shift occurs on the liquid crystal hologram170 or 171 even if the objective lens 4 changes its position to followtracks. Therefore, an offset of the TE signal or a reduction insensitivity of the FE signal can be prevented from being generated.

As the embodiments of FIGS. 19-21 allow the polarizing hologrambeamsplitter, such as element 170 or 171, to be positioned close to theobjective lens 4 and far from the photodetector units 100 and 101, theeffective diameter of the polarizing hologram beamsplitter can beenlarged even in the finite optical system, thus allowing a relativelylarge positional error in installing the polarizing hologrambeamsplitter into an optical head apparatus and, therefore, reducingcost of installation.

Furthermore, if the diffraction angle of the diffraction beam light isset small in the present invention, it becomes possible to dispose thephotodetector units 100 and 101 close to the radiation light source 2 soas to put it therebetween as shown in FIG. 22. In FIG. 22, referencenumerals 2a, 100a, and 101a denote mount members for mounting radiationlight source 2 and the photodetectors 100 and 101, respectively. Withthis arrangement, it becomes possible to eliminate mutual displacementof components due to temperature variation. And, therefore, thetemperature characteristics of the optical pickup head apparatus can befurther improved.

FIFTH EMBODIMENT

Moreover, FIG. 23 shows a fifth embodiment of the present invention. Inthis embodiment, all the optical components such as the radiation lightsource 2, the photodetector units 100 and 101, the liquid crystalhologram 170 and the quarter wavelength plate 15 (or the Faradayrotator), or the liquid hologram 171, and the objective lens 4 areassembled as a unit by an overall optical system holder 200.

With this arrangement, a mutual position between the objective lens 4and the radiation light source 2 is not varied even when the objectivelens 4 changes its position to follow the tracks. For this reason, anoccurrence of aberration can be further suppressed. Accordingly, theobjective lens 4 can be reduced its size and manufactured thin so as torealize a further small-sized, thin, optical pickup head apparatus.

As this invention may be embodied in several forms without departingfrom the spirit of essential characteristics thereof, the presentembodiments are therefore illustrative and not restrictive, since thescope of the invention is defined by the appending claims rather than bythe description preceding them, and all changes that fall within meetsand bounds of the claims, or equivalence of such meets and bounds aretherefore intended to embraced by the claims.

What is claimed is:
 1. An optical pickup head apparatus comprising:aradiation light source emitting a linearly polarized light beam; animaging optics receiving the light beam emitted from said radiationlight source, and converging the light beam to a small spot on aninformation medium; a plurality of photodetectors receiving the lightbeam being reflected and diffracted from the information medium throughsaid imaging optics and outputting electric signals in accordance withreceived light quantities; a hologram, having an anisotropic refractiveindex and disposed between said radiation light source and said imagingoptics, for transmitting therethrough in an undiffracted manner thelight beam emitted from said radiation light source and for diffractingthe light beam reflected from the information medium; a quarterwavelength plate, disposed between said hologram and said imagingoptics, for converting said linearly polarized light beam into acircular polarized light beam when said linearly polarized light beamtravels from said radiation light source to said information medium and,to the contrary, for returning the converted circular polarized lightbeam into a linearly polarized light beam when said circular polarizedlight beam comes back from said information medium; said photodetectorssensing light quantities of said light beam diffracted from the hologramso as to obtain an information signal, wherein a varying electricalsignal obtained from the photodetectors is used for obtaining theinformation signal, said imaging optics and said hologram are assembledin a holder in a rigid relation so as to be shifted together, and saidhologram is disposed close to said imaging optics and far from saidradiation light source.
 2. An optical pickup head apparatus inaccordance with claim 1, wherein, when said light beam converged ontosaid information medium is in a focus-meeting condition, one light beamdiffracted from the hologram converges in front of a correspondingphotodetector, while its conjugate light beam converges behind acorresponding photodetector.
 3. An optical pickup head apparatus inaccordance with claim 1, further comprising a collimator lens disposedbetween the hologram and the radiation light source.
 4. An opticalpickup head apparatus in accordance with claim 1, wherein the radiationlight source and the photodetectors are mounted close to each other as aunit in a package.
 5. An optical pickup head apparatus in accordancewith claim 1, wherein optical components including the radiation lightsource, the imaging optics, the photodetectors, the hologram and thequarter wavelength plate are assembled as a unit by an optical systemholder.